1. Field of the Invention:
This invention relates to a method and apparatus for severing formed articles from a sheet or web of thermoplastic material. More specifically, this invention relates to a method and apparatus for precisely locating an uncut thermoformed part by engaging small projections formed in the skeleton or scrap portion of the web using retractable male and female locators included as part of the punch and die respectively to precisely position the uncut article prior to severing.
2. Description of the Prior Art:
Methods and apparatus for forming uncut articles in a sheet or web of thermoplastic material are well known. Typically, a web of a thermoplastic material such as polystyrene is fed from a supply roll, through a pre-heating zone to give the material a suitable temperature for the forming process, and to a forming zone where a forming tool is used to form the article in the web. After cooling, the web of uncut articles are carried to a stamping tool which cuts the formed article out of the web. The various apparatus which may be used to form and trim such articles are extensive. For example, U.S. Pat. No. 3,518,334 to Carrigan et al and U.S. Pat. No. 4,477,243 to Wallsten both disclose apparatus for forming articles in a web of thermoplastic material.
Once formed, the uncut article should be carefully cut from the thermoplastic web with care and precision. Inaccurate trimming procedures may result in various undesirable results such as excess material at the edges of the cut article or destruction of the article during the severing process. Additionally, the method of locating and severing the formed articles must be consistent with industry needs to maintain high production rates.
Various methods and apparatus for advancing the web of formed articles and for severing the formed article from the web of thermoplastic material are known. Several of these methods include the use of projections on the web for aiding the advancing or severing of the formed articles. For example, the above-cited patent to Wallsten acknowledges that the production of projections on the sheet material during the forming operation is old in the art. Wallsten preferably utilizes the projections along the edges of the sheet to push or pull the sheet to the trim press for cutting. A gripping means utilizes the projections to supply the web to the stamping means. Wallsten is silent, however, with respect to the utilization of any such gripping means or projections for precisely locating the uncut articles prior to stamping. See also U.S. Pat. No. 3,785,762 issued to Butzko et al.
U.S. Pat. No. 4,430,914 issued to Keim discloses a rotary apparatus for advancing a web or formed thermoplastic articles. In Keim, rotary advancing means directly engage the web at spaced locations across the width of the web. A repeat indexing movement advances the web to the trimming station. The indexing movement is controlled by drive means which periodically rotates the advancing means to produce the aforementioned indexing movement. Release means periodically release the engagement of the rotary advancing means with the web to enable relaxation of stress developed in the web and to allow the interaction of the web and the guide means in the absence of constraint by the rotary advancing means.
The use of locating pins to precisely locate uncut articles with respect to the cutting or stamping apparatus are not as common in thermoplastic article severing applications. The use of such locatang pins are, however, are much more well known in metal stamping applications for die making or other punching processes. For example, Jones, Franklin, Die Desion and Diemaking Practice, The Industrial press, 1930, page 196 discloses a guide pin which aligns a metal blank with a die cut by insertion into a pierced hole in the metal blank. Similar processes may be seen by reference to U.S. Pat. No. 4,624,162 issued to Arnold which discloses a punch press for punching a series of openings in a strip of sheet metal stock. An opening in the sheet is used for positioning the sheet so that the strip is properly positioned for a punch operation. The punch press includes a pilot punch which penetrates an opening in the strip previously pierced by a pierce punch and is received in a opening in the second face of the punch for adjusting the positioning of the stock strip prior to the next punch operation.
U.S. Pat. No. 3,656,381 issued to Gilev et al discloses a punch press for punching sheet members from circular blanks. In the center of the punch press is a pin for centering the blank relative to the female die. The pin diameter corresponds to the size of the central opening of the blank fed into the die. The punching process transforms the blank into a rotor lamination for an induction motor.
U.S. Pat. No. 3,502,761 issued to Dimitracopoulos discloses the utilization of locating holes in either in the center of the product or spaced around the product in the web. In Dimitracopoulos, a piercing die penetrates the web to form a hole. A locating pin passes through the formed hole and mates with an opening in the second face to align the sheet prior to cutting a disc from the web.
The application of such processes for locating thermoplastic articles prior to severing have been much less successful. Such processes, which generally include the capture of a hole in a sheet for locating the sheet prior to stamping, are often unsuitable for thermoplastic operations. Thermoplastic sheets are much less stronger and would be less likely to withstand pressure exerted on the sheet by a locator without damage. Furthermore, thermoplastic applications often requires precise trimming of a pre-formed article from a web not required in many sheet metal stamping processes.
Despite such problems, limited applications of the use of projecting members in thermoplastic trimming apparatus where the projecting members engage the articles themselves are known. Such methods often rely on projecting members of dimensions close to the size of the articles themselves and on uniquely shaped articles which afford surfaces for easy engagement with the projecting member, e.g. cup-shaped articles. Other such applications utilize web materials or operating conditions which permit the projecting member to reshape the uncut articles prior to separation. For example, U.S. Pat. No. 3,461,756 issued to Mojonnier discloses a method for trimming plastic preforms from a sheet of plastic where the side walls of the preform are engaged between inner and outer shaping members to conform the preform to the preselected shape of the shaping member. Relative movement between the shaping members and a trim die while the shaping members are maintained in engagement with the preform punches the preform from the web of plastic material. U.S. Pat. No. 3,240,851 issued to Scalora discloses a method and apparatus for severing a cup-like article of thermoplastic material where a guide engages the inside of the formed article to center it normal to the sheet and make the sheet taut across a back-up member sized to encircle the formed article.
U.S. Pat. No. 3,523,474 issued to Kinslow discloses a method for trimming thermoformed parts by drawing a vacuum between the objects and a punch member to position the object on the punch prior to shearing the object from the sheet. The punch includes a protrusion inside the periphery of the cutting edge which interacts with a similarly shaped deformation on the periphery of the product. Suction draws the object to fit over the protrusion to closely align the punch and the object. Kinslow does not suggest, however, the use of a female protrusion in the aligning the object.
Also known in the art is the use of spring loaded male locators in combination with a punch for locating thermoplastic articles prior to cutting them from a web. The articles are formed with two small, narrow tip cone-shaped protrusions spaced on the inner sides of the outer four articles of a five article trim shot. The male locators positioned the web protrusions in complementary shaped female openings in the die face to locate the articles with respect to the cutting edge of the punch.